The present invention relates to an acceleration sensor element for three-dimensionally determining the direction and the rate of acceleration applied from the exterior using a piezoelectric body, to an acceleration sensor therewith, and to the manufacture thereof.
In the automobile industry and the machinery Industry demands for sensors capable of accurately determining physical quantities such as force, acceleration, and magnetic force are increasing. In particular, small sensors capable of detecting physical quantities for each component in two-dimensions or three-dimensions are required.
For example, a sensor in which a plurality of piezoelectric bodies are mounted on a flexible plate having an operation body is disclosed in Japanese Unexamined Patent Application Publication No. 5-26744.
The sensor is constructed so that the flexible plate may be deflected corresponding to physical quantities applied to the operation body from the exterior, and can three-dimensionally determine the direction and the rate of the physical quantities by electric charges generated in the piezoelectric body depending on the deflection of the flexible plate using a single sensor element.
The sensor element will be described using as an example, an acceleration sensor having a weight as the operation body. As shown in FIG. 2, when acceleration a is applied to the sensor element from the exterior, an inertial force f is applied to a weight 10 in a direction opposite to the acceleration a, and as a result, deflection 14 due to the inertial force f is generated at a flexible plate 12 horizontally disposed between the weight 10 and a support 11. Since electric charges depending on the direction and the rate of the deflection 14 are generated at a piezoelectric body 13 constructed over the flexible plate 12, the acceleration applied from the exterior can be three-dimensionally determined by measuring the electric charges.
In the sensor element as shown in FIGS. 3(a) and 3(b), the center of a base of the cylindrical weight 10, on which the flexible plate 12 is horizontally disposed, is specified as the origin O, a surface including the origin O, which is parallel to the flexible plate 12, is specified as an X-Y plane, the X-axis and Y-axis are specified so as to perpendicularly intersect each other in the X-Y plane, and the Z-axis is specified to Include the origin O and to perpendicularly intersect the X-Y plane.
In this instance, when it is assumed that a part sandwiched by one set of an upper electrode and a lower electrode of The piezoelectric body 13 is xe2x80x9cone piezoelectric element,xe2x80x9d for example, four elements of piezoelectric bodies and electrodes, each corresponding to the X-axis and the Y-axis, and eight elements of piezoelectric bodies and electrodes corresponding to the Z-axis can be arranged on the flexible plate 12.
In this case, the magnitude of each inertial force f applied to the weight 1 by the acceleration a applied from the exterior is determined as follows: the magnitude of an inertial force fX in the X-axis direction as shown in FIG. 4(a) is determined by the amounts of the electric charges generated at xe2x80x9cpiezoelectric elementsxe2x80x9d E1 to E4; the magnitude of an inertial force fY in the Y-axis direction is determined by the amounts at E5 to E8 (not shown); and the magnitude of an inertial force fZ in the Z-axis direction as shown in FIG. 4(b) is determined by the amounts at xe2x80x9cpiezoelectric elementsxe2x80x9d E9 to E12 and E13 to E16 (not shown).
In addition, the directions of the inertial forces are each determined from the polarity patterns of the electric charges (e.g., for a piezoelectric body top surface in FIG. 4(a), [+xe2x88x92+xe2x88x92] from the left; and for a piezoelectric body top surface in FIG. 4(b), [+xe2x88x92xe2x88x92+] from the left).
The inertial force f as a resultant force of fX, fY, and fZ determined as described above, and the direction and the rate of the acceleration a applied from the exterior can be three-dimensionally determined by a single small sensor element.
In the aforesaid sensor element, the entire flexible plate does not deflect uniformly; instead, the deflection stress rises rapidly, particularly at a boundary portion of the weight or the support and the flexible plate. Accordingly, when the piezoelectric elements are disposed at the boundary portion, the electric charges generated at the piezoelectric elements rapidly increase or decrease depending on slight deviations in the arrangement positions of the piezoelectric elements or variations in the shape of the weight or the support, so that it is possible to cause not only a variation of the output from each axis but also degradation in the sensor accuracy.
In addition, when the piezoelectric elements are disposed at a boundary portion, it is possible to cause degradation in sensor reliability due to the following deficiencies.
First, when the sensor element receives a rapid acceleration, a larger bending stress is applied to the boundary portion compared to other parts, and as a result, a particularly large force will be exerted not only on the piezoelectric elements but also on the piezoelectric body at the boundary portion Since the piezoelectric body Is made of piezoelectric ceramics such as PZT and PMN, whose mechanical characteristics such as strength and toughness are generally inferior, there is a concern that the piezoelectric body may be damaged. Second, since the behaviors of the pliable flexible plate and the rigid support or weight when the impact is applied are different, there is a concern that the piezoelectric elements may peel off from the flexible plate even if the piezoelectric body Is not damaged with the construction in which lower surfaces of the piezoelectric elements contact both the flexible plate and the support or the weight.
In addition, in the aforesaid sensor element, the flexible plate is required to have high flexibility in order to have sufficient sensitivity, while the weight and support are required to have high rigidity and to be unlikely to deflect in order to accurately detect the acceleration applied thereto.
If the sensor element is constructed by assembling the members such as the weight, the support, and the flexible plate, which were manufactured separately, the aforesaid opposed characteristics can be satisfied; however, there are problems in that many components and steps are required, resulting in lowered productivity.
As means for reducing the number of components and steps and for improving productivity, integral forming by the same material can be possible. When integral forming by ceramics is used as an example, the following methods may be mentioned: a ago method of cutting a ceramic green body 33 formed by filling a mold 30 with a ceramic power 31, and by conducting uniaxial pressing, etc., as shown in FIG. 5; a method of filling a mold 30 with a ceramic powder 31 and forming for a sensor 34, and then performing uniaxial pressing as shown in FIG. 6; a method of injection molding ceramic slurry 32 as shown in FIG. 7; and a method of molding by slip casting.
In any of the aforesaid methods, however, since the strength of the flexible part of the integral compact is extremely low and the dispersion of the density distribution is large, it is difficult to manufacture the flexible plate thinly and to precisely control the thickness of the flexible plate. That is, in the sensor element formed by the above method, the flexible plate is unlikely to deflect and the degree of deflection differs for each flexible plate or depending on a portion thereof.
Accordingly, in addition to the drawback of low sensitivity, there is a case in which the sensitivity differs for each sensor element, even if the same acceleration is applied, and also when the sensitivity differs for each axis, the direction and the rate of the acceleration obtained from the resultant force thereof are inaccurate, so that sensor accuracy is degraded.
The present invention is made in consideration of the above problems, and the objects thereof are to further improve sensor sensitivity and sensor accuracy while offering the advantages of a sensor element that is small and that can measure three-dimensional physical quantities using a single sensor element, and to provide a manufacturing method in which the sensor element can be easily manufactured.
The present invention provides an acceleration sensor element (hereinafter referred to as a xe2x80x9csensor elementxe2x80x9d) including a weight, a support having a hollow part, which is disposed around the weight as a center, and a flexible plate having piezoelectric elements in which a piezoelectric body is sandwiched by at least one set of electrodes. The flexible plate is disposed horizontally across the support so as to suspend the weight at the center of the hollow part of the support. Acceleration applied from the exterior is converted to a deflection of the flexible plate based on the behavior of the weight corresponding to the acceleration, and the direction and the rate of the acceleration are three-dimensionally determined by electric charges generated from the piezoelectric body corresponding to the deflection of the flexible plate. The piezoelectric body is disposed only at flexible parts of the flexible plate.
The sensor element according to the present invention is preferably a sensor element having the support the weight, and the flexible plate integrally sintered by a green sheet lamination technique. In addition, it is preferably an acceleration sensor element having the support, the weight, and the flexible plate integrally formed, in which the crystal structure or the crystal-grain diameter of at least one of the support, the weight, and the flexible plate differs from that of the others.
Furthermore, the sensor element according to the present invention is preferably a sensor element having a hole penetrating perpendicularly in the center of the weight so that the space above the flexible plate and the support hollow part may communicate with each other, in which narrow grooves communicating with at least the space at the side of a support outer periphery are formed at the positions on a support lower-end surface, which are each symmetrical with respect to the straight lines passing through the support center on the plane including the support lower-end surface, and in which the weight is suspended so that the plane including the support lower-end surface will not contact the lower-end surface thereof.
In addition, the present invention provides a manufacturing method for the acceleration sensor element, including a first step of laminating and compression bonding a green sheet of the configuration (in cross section) of the weight, the support and the flexible plate to form a laminated layer, a second step of integrally sintering the laminated layer to form a sintered body, and a third step of forming the piezoelectric elements on the sintered body by a thick-film method, and then sintering them.
In the manufacturing method according to the present invention, through holes are preferably punched in the center of the green sheet of the configuration in cross section of the weight and the flexible plate and at least two corners of the green sheet of the configuration in cross section of the support and the flexible plate, and laminating and compression-bonding are preferably conducted to make the laminated layer in a state where the through holes are penetrated by support pins.
In addition, in the manufacturing method according to the present invention, it is preferable that after the green sheet of the configuration in cross section of the weight, the support, and the flexible plate has been laminated and compression bonded on a planar green sheet to form a laminated layer, and then the laminated layer has been integrally sintered to form a sintered body, the sintered planar green sheet is cut in order that the lower end of the weight can be moved.
Furthermore, in the manufacturing method according to the present invention, it is preferable that after the planar green sheet has been placed on a green sheet of the configuration in cross section of the support, and furthermore the green sheet of the configuration in cross section of the weight, the support, and the flexible plate has been laminated and compression bonded to form a laminated layer, and then the laminated layer has been integrally sintered to form a sintered body, the sintered planar green sheet is cut in order that the lower end of the weight can be moved.
Furthermore, the present invention provides an acceleration sensor manufacturing method characterized in that the acceleration sensor element according to the present invention is fixed on a circuit board and is sealed by a cover in a dry atmosphere with a dew point of xe2x88x9240xc2x0 C. or less.